Variable inductance



July 18, 1961 c. R. ELLIS VARIABLE INDUCTANCE Filed Oct. 10, 1955 INVENTOR. CHARLES R/CHARD ELL/S nitecl States Patent O 2,993,182 VARIABLE INDUCTAN CE Charles Richard Ellis, Syracuse, N.Y., assign'or, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Oct. 10, 1955, Ser. No. 539,702 2 Claims. (Cl. 336-15) This invention relates to an improved variable-turn coil and more particularly to an improved continuously variable inductance or continuously tunable tank circuit adapted for power use in radio frequency equipment.

This invention is an improvement over the device shown and described in US. Patent 1,668,414.

An object of this invention is to provide an improved variable-turn coil adapted for use as a continuously variable inductance or a continuously tunable tank circuit.

A further object is to provide an improved calibrated continuously variable inductance adapted for high power use in radio frequency equipment and adapted to be varied at a relatively rapid rate.

A further object is to provide an improved continuously-variable-turn high power coil wherein the number of convolutions may be readily varied at relatively rapid rate for changing either inductance or tuned frequency and which in addition has a long life despite frequent rapid changes.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

The single figure is an isometric view of a preferred embodiment of this invention.

The preferred embodiment of the invention shown on the drawing includes a support having parallel elements 12 and 14 of insulating material secured thereto. The element 12 may include conventional anti-friction bearings, not shown, in the bores 16, 17, and 18 formed in element 12. The element 14 may include conventional antifriction bearings, not shown, in the bores 21 and 22 that are axially aligned with the bores 16 and 17, respectively, in the element 12. Ring conductors 24 and 25 each having inwardly projecting resilient contact fingers are secured to element 14 around the openings 21 and 22, respectively. Ring conductors 2'4 and 25 are shown on the outer side of element 14 for clarity in illustration; for practical reasons they may be disposed on the inner side of element 14. The ring conductor 21 may be formed with an insulating interruption, not shown, to prevent it from being a shorted secondary winding. However, ring conductor 24 need not have an insulating interruption if it is made of low loss conducting material, i.e., silver or copper. In fact, if ring 24 is not interrupted, the minimum inductance obtainable will be smaller than if ring conductor 24 is interrupted. This is due to the fact that ring conductor 24 is closely coupled to the first ribbon turn on coil 32 and the effect of a shorted turn coupled into an inductance is to decrease that inductance. If the ring conductor 24 is a good conductor, there is little power loss. Minimum inductance is obtained when all turns but one are on the metal cylinder 54. Input terminals 23 and 29 are connected to the ring conductors 24 and 25, respectively.

One end of a low loss circular dielectric cylinder 32 is supported in the bore 21 of element 14. An anti-friction bearing 34 is secured to the opposite end of cylinder 34. One end of a shaft 36 is rotatably supported in bearing 34 and the other end of shaft 36 is rotatably supported in the bore 16 of the element 12. A spur gear 38 is secured to shaft 36. A tension spring 42 is secured between spur gear 38 and bearing 34. An anti-fricti0n bearing 44 of insulating material is secured to the opposite end of cylinder 32.

A rigid coaxial conductor 46 terminating in output terminals 47 and 48 is supported by bearing 44 for longitudinal sliding movement coaxially of cylinder 32. Terminal 48 is usually at ground potential. The bearing 44 is of insulating material to avoid connecting the outer conductor of coaxial conductor 46 which is usually at ground potential to the ring conductor 24 which may be at high voltage. The ends of a coupling coil 52 disposed coaxially inside cylinder 32 are electrically connected to coaxial conductor 46. The coupling coil 52 is supported by the coaxial conductor 46.

One end of a conducting circular cylinder 54 whose outside diameter is identical to that of cylinder 32 is rotatably supported in the bore 22 of element 14, and is electrically contacted by the resilient fingers of ring conductor 25. One end of a shaft 56 is fixedly secured to the opposite end of cylinder 54. The other end of shaft 56 is supported for rotation in bore 17 of element 12. A spur gear 58, identical to spur gear 38, is secured to the shaft for meshing engagement with spur gear 38. A manually rotatable shaft 62 having a knob 63 is supported in the bore 18 of element 12. The shaft 62 need not be manually rotated; it is adapted to be coupled to a motor, not shown. One of the features of this invention is that it can be motor driven at speeds greater than 60 r.p.m. without damage. A pinion 64 is secured to the shaft 62 for meshing engagement with the spur gear 58. Mechanical indicator means of conventional design including a gear 65 and a pointer 65 is actuated by gear 64. Synchros may be used for indicating purposes instead of the mechanical indicating means.

A conducting ribbon 66 preferably of silver plated beryllium copper is secured at one end to one end of conducting cylinder 54 and is secured at its other end to the remote end of dielectric cylinder 32. The relatively large cross sectional area of the ribbon 66 provides for high current capacity. In addition the ribbon 66, being of beryllium copper, allows high speed mechanical operation. Other cross sections for the conductor 66, such as a square or a circle would fatigue rapidly under constant bending. in practice it was found that a ribbon of one one-hundredth inch thickness could be wound around a two inch diameter cylinder and not exceed its tensile strength limit; no other cross section would allow this. Since the limit it not exceeded, the ribbon can be wound on and off thousands of times without fatigue. No evidence of fatigue was seen in life tests. The stretch in the ribbon is insignificant under normal operation. The limiting number of winding operations of the variable inductance-tank resides in the mechanical system associated with the ribbon, e.g., gears and bearings.

Between its ends the conducting ribbon 66 is helically coiled about the cylinders 32 and S4 in opposite directions. The pitch of the helix wound on the dielectric cylinder 32 is varied. The spacing between turns is decreased continuously from the end near element 14 to the end near element 12. One manner of varying the pitch is set forth in US. Patent 1,936,671. A constant voltage can then appear across the coil. When there are many turns on the coil on dielectric cylinder 32, the voltage per turn is low and little spacing is needed between turns; when there are few turns, the voltage per turn is high and wider spacing between turns is necessary to avoid arcing. The end of the conducting ribbon which is secured to cylinder 32 is conductively attached to a coin silver ring 23 secured to the end of cylinder 32. The spring fingers of ring conductor 24 makes contact with the coin silver ring 23 and is adapted to slide relative to it. The coin silver ring 23 rotates with dielectric cylinder 32 and the ring conductor 24 remains fixed. Thus a rotating sliding contact is obtained which has much less residual inductance than a conventional slip ring and brush system.

When the conducting ribbon is coiled about the cylinders the bias spring 42 is sufliciently tensioned to ensure proper contact of the ribbon and cylinder 54 and to ensure that the conducting ribbon lays on and off the cylinders in the same position after many operations. This ensures that the calibration is reliable whether the device is used as a variable inductance or as a tuned tank circuit in the megacycle range.

To design a specimen of the invention, the inductance of the system may be estimated with the aid of the current sheet formula. Additionally, critical dimensions are involved and are concerned with the fact that when all the ribbon 66 is wound onto cylinder 32, cylinder 32 in conjunction with cylinder 54 can form a parallel transmission line. Care must be exercised to ensure that the quarter wave transmission line formed by the length of cylinder 54 or cylinder 32 is a quarter wave of a frequency that is much higher than the operating frequency of the system.

The structure described contains features that are similar to that disclosed in US. Patent 1,668,414. One important novel structural feature of this invention is the bias spring 42. A second important novel structural feature is flat ribbon for conductor 66. A third important novel structural feature is the longitudinally adjustable coupling coil 52. A fourth important novel structural feature are the high current capacity finger rings 24 and 25.

In operation the portion of the conducting ribbon 66 between the cylinders is under continuous substantially constant tension. The unwanted part of the conducting ribbon is coiled around the equipotential surface of conducting cylinder 54 and its wide flat surface and makes good contact with the equipotential surface under the influence of the tension applied by spring 42. Since the ribbon wound on the conducting cylinder 54 is completely shorted, no spurious voltages can appear across it. Thus the device may be used at high frequencies. To change the inductance or the tuned frequency of the coiled ribbon on cylinder 32, the manual knob or the motor means, not shown, is rotated to change the number of coils of the ribbon on the cylinder 32. The spring 42 serves the additional important function of causing the conducting ribbon to lay on and lay off of the cylinders in the same position lending reliability to the calibration unit 65-65. The inductance or frequency tuning between terminals 23 and 29 is determined essentially by the number of turns of ribbon on the dielectric cylinder and varies smoothly and continuously as the cylinders are rotated. The coupling coil 52 on the end of coaxial conductor 46 couples power from the coil on the cylinder 32 to utilizing devices. The coaxial conductor 46 is movable longitudinally whereby the coupling coil is adjustable longitudinally of the cylinder 32 to vary the coupling or to change its position in accordance with each change in the number of coils on cylinder 32 for obtaining maximum coupling. The minimum of the range of adjustment of inductance is determined by the spacing between the cylinders, the width of the ribbon and the thickness of the ribbon. The maximum of the range of adjustment of inductance is determined by the length of the cylinders and the number of turns on the dielectric cylinder.

Contact to cylinder 54 and end of ribbon 66 is through large area complete circle contact fingers capable of carrying high current.

A specimen of this invention has been constructed. It has operated satisfactorily for delivering 1,000 watts or more of radio frequency energy over a continuous frequency range of 15-60 megacycles as the tank circuit in a self-excited oscillator.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. A variable-turn coil of the type in which a conducting cylinder and a nonconducting hollow cylinder are disposed in side by side but spaced apart relation and are mounted for rotation about their respective axes, with an elongated flexible conductor attached at one end to one of the cylinders and partly wound tightly and progressively about the one cylinder and thence extending between the cylinders to the other cylinder and the remainder wound tightly and progressively about the other cylinder in the same axial direction and attached at its other end to the other cylinder, with means to rotate said cylinders to wind the conductor on either cylinder progressively and axially along it while unwinding it from the other cylinder, and with means to establish electrical connection to the ends of said conductor throughout rotation of said cylinders; the combination therewith of a rigid coil disposed inside said hollow nonconducting cylinder, rigid conductor means electrically connected to the rigid coil and mechanically connected coaxially therewith, bearing means secured to one end of said nonconducting hollow cylinder, said rigid conductor being slidable axially in said bearing means whereby said rigid coil may be axially adjusted inside said nonconducting hollow cylinder so as to vary the coupling or to change its position in accordance with each change in the number of turns on the nonconducting cylinder in order to obtain maximum coupling.

2. In a variable-turn coil of the type in which a conducting cylinder and a nonconducting cylinder are disposed in side by side but spaced apart relation and mounted for rotation about their respective axes, with an elongated flexible conductor attached at one end to one of the cylinders and partly wound tightly and progressively about the one cylinder and thence extending between the cylinders to the other cylinder and the remainder wound tightly and progressively about the other cylinder in the same axial direction and attached at its other end to the other cylinder, with means to rotate said cylinders to wind the conductor on either cylinder progressively and axially along it while unwinding it from the other cylinder, and with means to establish electrical connections to the ends of said conductor throughout rotation of said cylinders; that improvement in said means to establish electrical connections to the ends of said conductor which comprises a continuous ring conductor element with inwardly projecting resilient fingers surrounding said nonconducting cylinder and in continuous sliding electrical engagement with the end of said elongated conductor that is attached to said nonconducting cylinder whereby the lower limit of inductance obtainable when only one turn of said elongated conductor is on said nonconducting cylinder is minimized because said ring conductor is closely coupled to the first turn of said elongated conductor and the effect of a shorted turn closely coupled to an inductance coil is to decrease that inductance.

References Cited in the file of this patent UNITED STATES PATENTS 862,361 Thompson Aug. 6, 1907 1,080,271 Fessenden Dec. 2, 1913 1,740,850 Zarate Dec. 24, 1929 2,223,080 Swarbrick Nov. 26, 1940 2,666,905 Gerlach Jan. 19, 1954 

